You can choose whatever you want, including an Apple Music membership or anything from the iTunes Store, App Store, iBooks Store, and Mac App Store.
iTunes Terms and Conditions: Valid only on purchases made in the U.S. from the U.S. iTunes Store. Use requires an active iTunes account & prior acceptance of license & usage terms. Terms apply; see apple.com/legal/itunes/us/terms.html.
How to redeem: read on: https://support.apple.com/en-us/HT202644
iPhone, iPad, or iPod touch
1. Tap iTunes Store, iBooks, or App Store. In iBooks, then tap the Featured tab at the bottom of the screen.
2. Scroll to the bottom of the store and tap Redeem.
3. Sign in with the Apple ID that you use for iTunes Store purchases or your Apple Music membership.
4. Tap Use Camera. You can also redeem your code manually.
5. Peel off the label on the back of your gift card.
6. Hold the back of the card (the side with the black focus box) up to your camera.
7. The camera reads and redeems the code. Your iTunes Store balance appears onscreen, or you´re asked to apply the credit toward an Apple Music Individual Membership.
8. In the upper-right corner, tap Done.